1. Field of the Invention
The present invention relates to a dry etching method for forming a trench (deep groove), a deep hole, and the like in a silicon (Si) substrate.
2. Description of the Related Arts
Conventionally, a trench is formed in an Si substrate by an etching method to electrically isolate parts within a semiconductor element formed on the substrate. When forming the trench, a silicon dioxide (SiO.sub.2) layer is generally used as an etching mask. In this case, the etching process is performed under conditions such that the SiO.sub.2 layer is not easily etched, that is, an etching speed ratio of an etched field (Si substrate) with respect to the etching mask (SiO.sub.2), which is herebelow referred to as selectivity, is high. The silicon substrate is etched anisotropically in a depth direction thereof. As a result, a trench having a high aspect ratio can be formed in the silicon substrate.
In this method, however, reaction products which contain silicon and oxygen are produced during the etching process. The reaction products are liable to flocculate and deposit within an etching chamber. The deposited reaction products are further liable to be suspended within the etching chamber. In addition, SiO.sub.2 particles are separated from the etching mask. As a result, foreign materials which contain silicon and oxygen are attached to an etched portion of the silicon substrate, and serve as an etching mask. Then, there arises a problem inasmuch as non-etched portions, i.e., etching residues that are called black silicon (Si), are produced.
Processes for producing the etching residues are explained in more detail with reference to FIGS. 1A to 1C. As shown in FIG. 1A, a mask 1 is formed on an Si substrate 2 to be opened at a specific region, with an etched portion 6 formed in the mask 1. Then, the etched portion 6 is etched to form a trench 3. In this case, if a foreign material 5 is deposited on the trench 3 during the etching process as shown in FIG. 1B, the foreign material 5 serves as an etching mask in the successive etching process. As a result, a columnar black Si (etching residue) 4 is formed as shown in FIG. 1C. In a case where the black Si 4 is relatively large in size, an appropriate isolation width of the trench 3 cannot be obtained, so that it becomes difficult to obtain electrical isolation between elements formed on the substrate 2. As the amount of the foreign material 5 is increased, the amount of the black Si 4 is increased, resulting in an increased rate of occurrence of insulation deficiencies of the substrate 2. That is, the black Si 4 is easily generated in the trench 3 under etching conditions in which the reaction products are highly generated. In the above-mentioned trench etching process, because the etching process generates reaction products which contain silicon and oxygen, the probability of producing the black Si is high.
In addition, conventionally, a peripheral portion of the substrate within several millimeters from an outer circumference of the substrate is not covered with the SiO.sub.2 etching mask, so that the etching process is performed in a state in which the peripheral portion of the substrate is exposed. Therefore, the black Si is generated on the peripheral portion of the substrate having a width larger than that of the trench. The black Si on the peripheral portion of the substrate can be a foreign material source in the successive etching process.
To solve this problem, the SiO.sub.2 mask may be left on the peripheral portion of the substrate. However, in such a case, a ratio of an area of the etched portion of the substrate with respect to the entire area of the substrate becomes small, so that the etching speed of the SiO.sub.2 mask is increased to decrease the amount of the reaction products, resulting in low selectivity. Therefore, the etching conditions need to be changed so that the selectivity becomes large to prevent the etching of the SiO.sub.2 mask. This means changing the etching conditions to increase the amount of the reaction products. For example, a flow rate of silicon tetrafluoride (SiF.sub.4) gas and oxygen (O.sub.2) gas, which are used as an etching gas in a mixed state, is increased.
On the other hand, there is a tendency for the edge portion of the mask to be etched by a sputter-etching effect as it becomes close to the outer circumference of the substrate. That is, the larger the diameter of the substrate becomes, the easier the edge portion of the mask is etched. Therefore, when the diameter of the substrate is large, the etching conditions need to be changed so that the selectivity is increased to prevent the mask. However, as mentioned above, increasing the selectivity causes an increase to the amount of the reaction products generated. As a result, the amount of foreign materials attached to the etched portion of the substrate is increased so that the amount of the black Si generated is increased, resulting in isolation deficiencies.
When the generation of the reaction products are suppressed to prevent the occurrence of the black Si, the selectivity becomes small, and when the selectivity is controlled to be large, the amount of the reaction products is increased. An etching method for solving the thus colliding problems is disclosed, for example, in JP-A-8-17804. In the etching method, in a state where an Si substrate that is to be etched (etched substrate) is disposed within an etching chamber, a cleaning process using a gas containing fluorine (F) is performed generating plasma discharge. Accordingly, an exposed naturally oxidized layer on the Si substrate and reaction products that are produced in a previous etching process and deposited within the etching chamber are removed. Then, the next etching process using a gas including chlorine (Cl), iodine (I), and bromine (Br) gases is performed with respect to the Si substrate to form a trench in the substrate. In this case, a shape of the trench is improved by removing the naturally oxidized layer of the substrate. In addition, deposition of the reaction products on the substrate is suppressed by removing the reaction products within the etching chamber.
However, in the above-mentioned etching method, the cleaning process using the gas containing F is performed while the etched substrate is disposed within the etching chamber. Therefore, the shape of the trench in the substrate is affected by the cleaning process, and accordingly, the trench cannot be formed with high accuracy.